The article “Dual Source CT: Detecting Urinary Stones by Spiral Dual Energy Computed Tomography With Virtual Non-Enhanced Images” by Paul Stolzmann et al., SOMATOM Sessions, pages 49 to 51, November 2007 discloses a dual-energy computed tomography apparatus comprising two x-ray tubes emitting x-rays with different energy spectra. The x-rays, after having traversed a region of interest, are detected by a detector, while the two x-rays tubes are rotated around the region of interest for illuminating the region of interest in different directions. The detector generates two sets of detection values, which correspond to the two x-ray spectra of the two x-ray tubes, and a reconstruction unit reconstructs two images of the region of interest from the two sets of detection values. This dual-energy computed tomography apparatus is technically relative complex.